1. Field of the Invention
The present invention relates to apparatus for the rapid and accurate determination of the center of interference fringes commonly encountered in optical metrology. Specifically, the apparatus determines the center of a fringe when the fringe pattern and a photoelectric sensor are scanned relative to each other. More particularly, the invention relates to noncontacting electro-optical apparatus for this type of determination which is capable of rapidly and accurately measuring the coordinates of fringe centers in either a real-time interference pattern or an interferogram.
Inteferometric testing has long been used in optical metrology. The advent of the laser has not only made interferometers more convenient to use but has also extended their range of application. Interferometry is used as a tool in optical fabrication, final testing, and system alignment, see for example, C. Zanoni, "Interferometry," The Optical Industry and Systems Directory Encyclopedia, v. 2, pp. E137-E141 (1977).
For most interferometric measurements, the information is contained in either a real-time inteference fringe pattern or an interferogram, i.e., a photograph of an interference pattern. The quantitative usefulness of an interference pattern is dependent upon having a method of data extraction and reduction. For a preliminary evaluation, positional deviations of the fringes can be obtained using a variety of simple manual techniques, see, for example, R. Berggren, "Analysis of Interferograms," Optical Spectra, pp. 22-25 (December 1970).
In order to extract information from either a real-time interference pattern or an interferogram for a more thorough evaluation, it is necessary to know the two-dimensional coordinates for an array of points located on the centers of the fringes.
The measurement of fringe centers on interferograms has been carried out using a variety of techniques. Most of the techniques use mechanical scanning to produce photoelectric signals whose equality is the signature for the location of a fringe center, see, for example, G. D. Dew, "A Method for the Precise Evaluation of Interferograms," J. Sci. Instr. 41, pp. 160-162 (1964) and J. Dyson, "The Rapid Measurement of Photographic Records of Interference Fringes," Appl. Opt. 2, pp. 487-489 (1963). Such fringe scanning techniques are capable of measuring fringe displacements of less than 0.01 fringe.
Another approach used with interferograms locates the center of the optical density curve by using a computer-generated fit to the output of a microdensitometer trace across a fringe, see, for example, R. A. Jones and P. L. Kadakia, "An Automated Interferogram Technique," Appl. Opt. 7, pp. 1477-1482 (1968). A microdensitometer is capable of measuring fringe displacement somewhat more accurately than 0.01 fringe.
Another technique used in an instrument manufactured by the assignee of this application is based upon using an oscillating spot of light to measure optical density gradients on an interferogram. The signature for sensing the location of an interference fringe center is the null in the first derivative of the optical density. Using an oscillating spot of light and synchronous demodulation leads to a considerably simpler instrument which achieves improved precision in the location of fringe centers with a minimum of equipment. However, this technique is extremely slow and, therefore, lends itself only to the measurement of interferograms. Furthermore, it is costly and difficult to automate this approach.
In order to measure real-time interference patterns without introducing errors and complexity, it is desirable to extract all of the fringe center data very rapidly, i.e., in a small fraction, 1/30- 1/60 , of a second, because of the fluctuations induced in the pattern by mechanical vibrations and atmospheric turbulence effects.
Sophisticated, expensive interferometers have been designed and built for the high precision, automatic reduction of real-time interference patterns. One such instrument is disclosed in Gallagher, et al., U.S. Pat. No. 3,694,088 issued Sept. 26, 1972. Another sophisticated digital interferometer is discussed in J. H. Bruning, et al., "Digital Wavefront Measuring Interferometer for Testing Optical Surfaces and Lenses," Appl. Opt. 13, pp. 2693-2703 (1974). These instruments, however, cannot reduce interferograms.
While these prior-art techniques for sensing the coordinates of the centers of fringes are useful for some applications, they cannot be used for many industrial applications. For example, in the manufacture of high precision, high volume optical components, interferograms and interferometer interference patterns must be measured in large numbers and at high speed with affordable instrumentation.
In my copending application Ser. No. 788,736 filed Apr. 19, 1977, I provide an apparatus for measuring interferograms comprising (1) a source of radiant energy either in the form of a beam of light containing an interference pattern or as illumination for an interferogram, (2) means for scanning the interference pattern, (3) means for collecting and photosensing the radiant energy, and (4) means for processing the photosensor outut to produce a signature indicating the center of a fringe accurately by defining the occurrence of a dark (or bright) fringe center as the instant at which the first derivative of the photosensor output is zero and, simultaneously, the magnitude of the second derivative of the photosensor output is negative (or positive) and below (or above) a preset threshold, and providing means to perform the necessary computations.
In working with this device, I discovered that the noise and time delays involved in using the second derivative presented very serious reliability and speed problems. It is desirable to sense the fringe centers reliably in a totally automatic manner. Moreover, with real-time fringe patterns, it is desirable to complete the operation in the time interval of one TV field, i.e., 1/60 second.